Supercritical water, a mysterious state where water behaves like both a liquid and gas, has long puzzled scientists.
Researchers in Germany used terahertz spectroscopy and powerful simulations to finally debunk a key theory—that water molecules form hydrogen-bonded clusters in this state. Building a specialized high-pressure cell, the team showed that supercritical water behaves much like gas, lacking the long-lasting hydrogen bonds found in liquid water. Simulations confirmed that interactions between molecules are fleeting and disordered.
Shedding Light on Supercritical Water
Researchers at Ruhr University Bochum in Germany have gained new insights into the structure of supercritical water, a unique state in which water behaves like both a liquid and a gas. This state occurs only under extreme conditions: temperatures above 374°C and pressures above 221 bar.
One long-standing theory suggested that in this state, water molecules form clusters held together by hydrogen bonds. However, the Bochum team has now disproven this idea using a combination of terahertz spectroscopy and molecular dynamics simulations. Their findings were published on March 14, 2025, in the journal Science Advances.
Why Supercritical Water Matters
Supercritical water can be found in nature, such as around deep-sea hydrothermal vents known as black smokers, where extreme pressure and heat exist. Understanding its molecular structure could help scientists better explain the chemistry that occurs in these environments.
“Understanding the structure of supercritical water could help us to shed light on chemical processes in the vicinity of black smokers,” says Dominik Marx, referring to a recent paper published by his research group on this topic.
“Due to its unique properties, supercritical water is also of interest as a “green” solvent for chemical reactions; this is because it is environmentally friendly and, at the same time, highly reactive.”
In order to improve the usability of supercritical water, it is necessary to understand the processes inside it in greater detail. Martina Havenith’s team used terahertz spectroscopy for this purpose. While other spectroscopy methods can be employed to investigate H-bonds within a molecule, terahertz spectroscopy sensitively probes the hydrogen bonding between molecules – and thus would allow to detect the formation of clusters in supercritical water, if there are any.
Engineering a High-Pressure Breakthrough
“In experimental trials, applying this method to supercritical water was a huge challenge,” explains Martina Havenith. “We need ten-fold larger diameters for our high-pressure cells for terahertz spectroscopy than in any other spectral range because we work with longer wavelengths.” While working on her doctoral thesis, Katja Mauelshagen spent countless hours designing and building a new, suitable cell and optimizing it so that it could withstand the extreme pressure and temperature despite its size.
Rewriting the Molecular Picture
Eventually, the experimentalists managed to record data from water that was about to enter the supercritical state, as well as from the supercritical state itself. While the terahertz spectra of liquid and gaseous water differed considerably, the spectra of supercritical water and the gaseous state looked virtually identical. This proves that the water molecules form just as few hydrogen bonds in the supercritical state as they do in the gaseous state. “This means that there are no molecular clusters in supercritical water,” concludes Gerhard Schwaab.
A member of Dominik Marx’s team, Philipp Schienbein, who calculated the processes in supercritical water using complex ab initio molecular dynamics simulations as part of his doctoral thesis, came to the same conclusion. Just like in the experiment, several hurdles had to be overcome first, such as determining the precise position of the critical point of water in the virtual lab.
Ephemeral Bonds Define Supercritical Water
The ab initio simulations ultimately showed that two water molecules in the supercritical state remain close to each other only for a short time before separating. Unlike in a hydrogen bond, the bonds between hydrogen and oxygen atoms don’t have a preferred orientation, which is a key property of hydrogen bonds. The direction of the hydrogen-oxygen bond rotates permanently.
“The bonds that exist in this state are extremely short-lived: 100 times shorter than a hydrogen bond in liquid water,” stresses Philipp Schienbein. The results of the simulations matched the experimental data perfectly, providing now a detailed molecular picture of the structural dynamics of water in the supercritical state.
Reference: “Random encounters dominate water-water interactions at supercritical conditions” by Katja Mauelshagen, Philipp Schienbein, Inga Kolling, Gerhard Schwaab, Dominik Marx and Martina Havenith, 14 March 2025,Science Advances.
DOI: 10.1126/sciadv.adp8614
The study was a collaboration between experimental physicists Dr. Katja Mauelshagen, Dr. Gerhard Schwaab, and Professor Martina Havenith from the Chair of Physical Chemistry II, and theoretical chemists Dr. Philipp Schienbein and Professor Dominik Marx from the Chair of Theoretical Chemistry. The work was supported by the Cluster of Excellence Ruhr Explores Solvation (RESOLV).
